[PATCH 2/3] introduce probe_slab_address()

[Oleg Nesterov <oleg@redhat.com>]

Extract the ifdef(CONFIG_DEBUG_PAGEALLOC) code from get_freepointer_safe()
into the new generic helper, probe_slab_address(). The next patch will add
another user.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/uaccess.h |   15 +++++++++++++++
 mm/slub.c               |    6 +-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index effb637..3367396 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -71,6 +71,21 @@ static inline unsigned long __copy_from_user_nocache(void *to,
 	__probe_kernel_read(&(retval), (__force void *)(addr), sizeof(retval))
 
 /*
+ * Same as probe_kernel_address(), but @addr must be the valid pointer
+ * to a slab object, potentially freed/reused/unmapped.
+ */
+#ifdef CONFIG_DEBUG_PAGEALLOC
+#define probe_slab_address(addr, retval)	\
+	probe_kernel_address(addr, retval)
+#else
+#define probe_slab_address(addr, retval)	\
+	({							\
+		(retval) = *(typeof(retval) *)(addr);		\
+		0;						\
+	})
+#endif
+
+/*
  * probe_kernel_read(): safely attempt to read from a location
  * @dst: pointer to the buffer that shall take the data
  * @src: address to read from
diff --git a/mm/slub.c b/mm/slub.c
index 3e8afcc..0467d22 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -265,11 +265,7 @@ static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
 {
 	void *p;
 
-#ifdef CONFIG_DEBUG_PAGEALLOC
-	probe_kernel_read(&p, (void **)(object + s->offset), sizeof(p));
-#else
-	p = get_freepointer(s, object);
-#endif
+	probe_slab_address(object + s->offset, p);
 	return p;
 }
 
-- 
1.5.5.1